Today I was installing Ubuntu on my new laptop (my Compaq F700 was sadly destroyed by extensive hardware failure for no apparent reason; the motherboard, hard drive and possibly everything else is all gone. It's been replaced by a shiny new Asus K60). I missed grabbing the GPG key for a new source I was installing. Rather than go track that down I found a more general solution for any missing GPG key.
So when Ubuntu tells you:

GPG error: http://ppa.launchpad.net karmic Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY [hex string giving the id of the key]